1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the invention relates to a high-resolution, high-brightness liquid crystal color display device based on a field sequential system and a driving method for the same.
2. Description of the Related Art
Liquid-crystal display devices (hereinafter abbreviated LCDs) have found widespread commercial applications in a variety of fields ranging from calculators to portable television sets (hereinafter abbreviated TVs) because of their excellent display performance rivaling that of the cathode ray tube (CRT herein after), their space-saving features exemplified by thin and light-weight construction, and other useful features such as low power consumption. While there are problems yet to be resolved, especially in response time and viewability, various improvements have been made in LCD technology because the LCD is a promising display device that is expected to replace the CRT in the near future. Among such improvements, improvements in color LCD technology involve various aspects of display performance and assume an important position in the development of the technology.
The principle of a color display is based on the method called "additive color mixing process". When two or more colored light beams enter the human eye, the light beams are combined on the retina and perceived as different colors. Based on this principle, any color can be obtained by additively mixing light beams of the three primary colors, R (red), G (green), and B (blue) in appropriate proportions. A color display in practical display devices is implemented using one of two systems based on the principle of the additive color mixing process.
One is the National Television System Committee (NTSC) system that uses a principle called "juxtapositional additive color mixing process". In this system, tiny color filters are placed close together in a matrix array in the display area of a single-plate display panel. These color filters are smaller in area than the spatial resolution limit of the human eye so that a combination of tiny color spots is perceived as a color by the eye. The NTSC system is compatible with monochrome television and currently, is the standard system for use in color TVs. However, in this "juxtapositional additive color mixing process", the R, G, and B primary colors become visible as separate colors unless the pixel size is smaller than the spatial resolution limit of the human eye. The juxtapositional additive color mixing process therefore poses a problem in that it reduces image quality when it is employed in a projection LCD or the like which projects an enlarged image for display.
The other system is one that employs a "simultaneous additive color mixing process". To apply this system for a color LCD, three color filters of R, G, and B are used in combination with three LCD panels, and three color images are simultaneously projected onto a screen where the color images are superimposed and merged into one color image. This system eliminates the fabrication difficulty of tiny color filters which is required in the Juxtapositional additive color mixing process. However, if there is a defective pixel in any of the three LCD panels, one of the R, G, and B colors, or a mixed color thereof, appears as a bright spot at the affected pixel position, thus making the defect noticeable. Furthermore, the provision of three LCD panels leads to increased size and cost of the display system.
Color LCDs have the above-mentioned shortcomings. In addition for the demand to overcoming these shortcomings, there is an increasing demand to enhance the resolution and brightness of color LCDs, which is imperative among others in the implementation of high-definition TV as the next-generation visual medium.
Higher resolution and higher brightness are conflicting requirements. On one hand, increasing the pixel density for increased resolution increases the ratio of the switching element area to the pixel area, which results in a reduction in the aperture ratio and consequently, a reduction in brightness. Conversely, if the aperture ratio is to be increased, the pixel area must be increased, which reduces the resolution. While the NTSC system is the standard system for color television today, the field sequential color system has now received renewed attention as a color system to overcome the above problems, for the reasons hereinafter described. The field sequential color system provides the following advantages in terms of high resolution and high brightness characteristics.
(1) The field sequential system uses a principle called "successive additive color mixing process". This process utilizes the resolution limit of the human eye in the time domain. More specifically, this process utilizes the phenomenon that when successive color changes are too fast for the human eye to perceive, the persistence of the previous color causes the color to be mixed with the succeeding color and these colors are combined and perceived as one color to the human eye. As in the simultaneous additive color mixing process, any desired color can be obtained at each pixel, so that the system achieves high image definition and also provides excellent color reproduction. The first color TV standard system utilized the field sequential system.
(2) If there is a defective pixel in the LCD panel, the affected pixel appears as black or white, which is not as noticeable as a colored bright spot. Therefore, pixel defects, up to a certain degree, will not lead to image quality reduction.
(3) Full-color or multi-color images can be displayed using a single LCD panel, which serves to reduce the size and weight of the display system. No cost increase is involved since no more than one LCD panel is required, unlike the simultaneous additive color mixing process which requires the provision of more than one LCD panel.
Color technology based on the field sequential system will be described below. FIG. 19 shows a color filter plate capable of high-speed sequential switching of colors. In the figure, a cyan filter 29C, a magenta filter 29M, and a yellow filter 29Y are formed one on top of another in this order.
The cyan filter 29C includes a pair of transparent substrates 20 and 21 with transparent electrodes (not shown) formed over the entire areas of the opposing surfaces thereof, and a liquid crystal layer 22 including a liquid crystal and cyan dichroic dye, sandwiched between the two substrates 20 and 21.
The magenta filter 29M includes a pair of transparent substrates 23 and 24 with transparent electrodes (not shown) formed over the entire areas of the opposing surfaces thereof, and a liquid crystal layer 25 including a liquid crystal and magenta dichroic dye, sandwiched between the two substrates 23 and 24.
The yellow filter 29Y includes a pair of transparent substrates 26 and 27 with transparent electrodes (not shown) formed over the entire areas of the opposing surfaces thereof, and a liquid crystal layer 28 including a liquid crystal and yellow dichroic dye, sandwiched between the two substrates 26 and 27.
The cyan filter 29C, magenta filter 29M, and yellow filter 29Y are each supplied with an AC voltage from an AC power supply 31, via their associated switching circuits 30C, 30M, and 30Y, respectively. Based on a select signal supplied from a display control circuit 16, the switching circuits 30C, 30M, and 30Y selectively apply the AC voltage to the cyan filter 29C, magenta filter 29M, and yellow filter 29Y, to drive the respective filters.
By controlling the activation and deactivation of each filter in this manner, light beams of the three primary colors, i.e., a red colored light beam, a green colored light beam, and a blue colored light beam are produced. Table 1 below shows the combinations in which the filters are turned on or off, in relationship to the resulting colors of incident light beam.
TABLE 1 ______________________________________ Combination Resulting 29C 29M 29Y colors ______________________________________ ON OFF OFF Red OFF ON OFF Green OFF OFF ON Blue ______________________________________
The operation of the field sequential color system using the above color filters will be described in detail below. FIG. 20 shows a timing chart for explaining the basic operation of a light beam selecting element 15. As shown, a voltage is applied to the cyan filter 29C during the period from time t1 to time t3. The orientation of the liquid crystal does not change immediately upon voltage application, but it takes a prescribed transition period .tau.. The transition period .tau. corresponds to the response time of the liquid crystal molecules to the applied electric field. Accordingly, even if voltage application is started at time t1, the liquid crystal in the cyan filter 29C does not immediately change the orientation in response to the applied voltage and the changed orientation does not settle down until time t2, i.e., until after the transition period .tau. has elapsed. As a result, the light beam selecting element 15 transmits a red colored light beam during a period TR starting at time t2 and lasting until time t3.
In like manner, voltage is applied to the magenta filter 29M, yellow filter 29Y, and cyan filter 29C in sequence, the light beam selecting element 15 transmitting a green colored light beam, blue colored light beam, and red colored light beam, respectively.
The light beam selecting element is not limited to the illustrated construction. It will be recognized that there are other possible constructions that can produce a desired color. For example, a construction including three kinds of liquid crystals containing red, blue, and green dichroic dyes, a construction including a liquid crystal panel combined with color polarizers, or a construction including a liquid crystal panel combined with neutral polarizers may be used.
Color technology based on the field sequential system has been described above. As described earlier, according to the field sequential system, a high-resolution, high-brightness color LCD having excellent image display quality can be achieved with a compact and light-weight construction.
However, LCD implementation of color display based on the field sequential system demands the following.
(1) Increased LCD response speed and stability of signal retention.
(2) Increased operating speed of switching elements.
Description is first given of (1) the increased LCD response speed and the stability of signal retention. FIG. 21 shows the equivalent circuit of a conventional liquid crystal driving circuit for each unit pixel in an active-matrix liquid crystal display device constructed with thin-film transistors (hereinafter abbreviated TFTs). The driving circuit shown includes a TFT 103, a pixel electrode 107, a liquid crystal capacitor LC, a counter electrode 108, and an additional capacitor Cs. The TFT 103 has a gate electrode 104 connected to a scanning line 101, a source electrode 105 connected to a data line 102, and a drain electrode 106 connected to the pixel electrode 107 and the additional capacitor Cs. A data signal corresponding to an image to be displayed is applied to the data line 102, and the signal is written to the pixel when the pixel is selected by applying a scanning signal to its associated scanning line 101. More specifically, when a scanning signal is applied to the scanning line 101, the TFT 103 connected to the scanning line 101 is turned on to selectively drive the pixel electrode 107. A voltage is applied between the selected pixel electrode 107 and the counter electrode 108, and the data signal is written as an electric charge on the liquid crystal capacitor LC between the two electrodes 107 and 108 as well as on the additional capacitor Cs.
In a display device having the liquid crystal driving circuitry as described above, if the minimum frame switching frequency at which the flicker is not perceivable by the human eye is 30 Hz or more, it follows that images in the R, G, and B primary colors must be displayed successively within 1/30 second, which is one frame period, in order to achieve full color display in accordance with the field sequential color system. These three images are merged using the retentivity of the human eye and as a result perceived as a full-color image. More specifically, if the display frequency is 30 Hz, then the images in the R, G, and B primary colors must each be displayed at a frequency of 90 Hz, which means that the LCD must display each color image in about 11 milliseconds. The LCD must be capable of displaying a good quality image within this period. This also means that the stored data signal must be retained in a stable state during the 11-millisecond period. Furthermore, to display 1125 scanning lines used in the High-Vision system, an extremely fast response is required. That is, a scanning signal must be applied to every one scanning line in about 10 microseconds.
Next, description is given of (2) the increased operating speed of the switching elements that is required of the liquid crystal driving circuitry employing the field sequential color system.
To produce images for ordinary High-Vision broadcasts, 1125 scanning lines and 1875 data lines are needed. In this case, the operating speed of about 102 KHz is required to the switching elements in the driving circuit for driving the scanning lines, and the operating speed about 190 MHz or more is required to the switching elements in the driving circuit for driving the data lines.
Thus, very fast switching elements are needed to implement the color display according to the field sequential system.
Materials for switching elements required to achieve such high-speed switching operations will be described below.
Liquid-crystal display devices usually use glass substrates. In active-matrix LCDs, switching elements such as TFTs are formed on such glass substrates. The characteristics of TFTs are determined by the kinds of thin films used to form the TFTs. The materials commonly used for the thin films are generally classified into one of the following three categories.
(1) Amorphous silicon PA1 (2) Low-temperature polysilicon PA1 (3) High-temperature polysilicon PA1 a first substrate having a single-crystalline silicon layer on one surface thereof, PA1 a transparent second substrate disposed opposite the first substrate, the surface of the first substrate having the single-crystalline silicon layer thereon facing the second substrate with a ferroelectric liquid crystal layer sandwiched therebetween, PA1 scanning lines and signal lines formed in the single-crystalline silicon layer in such a manner as to form a matrix, PA1 a first switching element, a second switching element, and a storage capacitor formed in the single-crystalline silicon layer in each of a plurality of pixel areas formed in the matrix, PA1 a pixel electrode deposited on a protective film formed over the entire surface of the single-crystalline silicon layer of the first substrate and covering the scanning lines, the signal lines, the first switching element, the second switching element, and the storage capacitor, the pixel electrode being provided in each of the plurality of pixel areas, with the first switching element being connected to the associated scanning line and signal line as well as to one electrode of the storage capacitor and the second switching element, and the second switching element being connected to the one electrode of the storage capacitor and the pixel electrode, and PA1 a transparent counter electrode deposited on the surface of the second substrate facing the first substrate. PA1 during the OFF period of the first switching element, the second switching element is held ON using the data signal retained in the storage capacitor during the ON period of the first switching element, thereby allowing a voltage from the power supply to be applied across the ferroelectric liquid crystal between the pixel electrode and the transparent counter electrode so that the ferroelectric liquid crystal layer is held at the substantially same potential as when the first switching element is ON.
Explanation will be given below of the thin films formed of the respective materials.
(1) Since amorphous silicon thin films can be formed at a relatively low temperature of about 350.degree. C., these thin films can be formed on a low-cost glass substrate, for example, a substrate made of Corning 7059 manufactured by Corning Ltd. However, ordinary low-cost glass cannot be subjected to temperatures of not lower than 600.degree. C. Therefore, a thermal oxide film having high insulating strength and high resistance to pinhole formation cannot be grown on a substrate made of such glass. In addition, there are many trapping states in an amorphous silicon thin film. For instance, the field-effect mobility .mu.e of an amorphous silicon thin film is about 0.1 to 0.5 cm.sup.2 V.sup.-1 S.sup.-1. Accordingly, amorphous TFTs formed on a low-cost glass substrate have a large ON resistance, which means that circuits, such as driver circuits, requiring complex and high-performance transistors cannot be fabricated on the same substrate as the display part.
(2) Low-temperature polysilicon is crystallized by long-period annealing or laser annealing. The maximum processing temperature is 550.degree. to 600.degree. C. Since polysilicon TFTs are formed at higher temperatures than amorphous silicon TFTs, polysilicon TFTs generally have good transistor characteristics. That is, the field-effect mobility .mu.e (electron mobility) is about 50 cm.sup.2 V.sup.-1 S.sup.-1, and .mu.h (hole mobility) is about 15 cm.sup.2 V.sup.-1 S.sup.-1.
(3) Since high-temperature polysilicon can be processed at temperatures as high as 1200.degree. C. when formed on a quartz substrate having excellent heat resistance, TFTs formed of high-temperature polysilicon have the best characteristics among the three categories of TFTs. A field-effect mobility .mu.e of about 100 cm.sup.2 V.sup.-1 S.sup.-1 can be obtained. Since TFTs having better characteristics than amorphous TFTs can be obtained, polysilicon thin films have the advantage that an IC fabrication process can be used for thin film fabrication and that some of driving circuits can be formed on the same glass substrate as the display part.
However, transistors formed of polysilicon, not to mention transistors formed of amorphous silicon, are slow with respect to operating speed. When the maximum operating frequencies are measured, for example, on CMOS shift registers formed from TFTs, the results are typically 5 MHz for low-temperature polysilicon TFTs, and 15 MHz even for high-temperature polysilicon TFTs. These operating speeds are slower than those needed to realize a color LCD based on the field sequential system. Therefore, TFTs capable of higher operating speeds are in high demand. Furthermore, since polysilicon TFTs have relatively large leakage currents, the TFT size has to be increased to provide a larger on/off ratio, or TFTs have to be connected in series. This makes the reduction of the LCD size difficult.